What is an SCC?

An SCC (Strongly Connected Component) is a maximal set of nodes where every node can reach every other node through directed paths. Maximal means you cannot add any more nodes to the set without breaking the mutual reachability property. If node $v$ is outside the SCC, either $v$ cannot reach the SCC or the SCC cannot reach $v$.

Example: In graph $1 \to 2 \to 3 \to 1$, nodes $\{1, 2, 3\}$ form an SCC because $1$ reaches $2$ reaches $3$ reaches $1$. Every pair is mutually reachable.